1. Field
The present invention relates to computer systems and methods in which data resources are shared among concurrent data consumers while preserving data integrity and consistency relative to each consumer. More particularly, the invention concerns improvements to a “read-copy update” (RCU) variant known as sleepable RCU or SRCU.
2. Description of the Prior Art
By way of background, read-copy update is a mutual exclusion technique that permits shared data to be accessed for reading without the use of locks, writes to shared memory, memory barriers, atomic instructions, or other computationally expensive synchronization mechanisms, while still permitting the data to be updated (modify, delete, insert, etc.) concurrently. The technique is well suited to multiprocessor computing environments in which the number of read operations (readers) accessing a shared data set is large in comparison to the number of update operations (updaters), and wherein the overhead cost of employing other mutual exclusion techniques (such as locks) for each read operation would be high. For example, a network routing table that is updated at most once every few minutes but searched many thousands of times per second is a case where read-side lock acquisition would be quite burdensome.
The read-copy update technique implements data updates in two phases. In the first (initial update) phase, the actual data update is carried out in a manner that temporarily preserves two views of the data being updated. One view is the old (pre-update) data state that is maintained for the benefit of read operations that may have been referencing the data concurrently with the update. The other view is the new (post-update) data state that is available for the benefit of other read operations that access the data following the update. These other read operations will never see the stale data and so the updater does not need to be concerned with them. However, the updater does need to avoid prematurely removing the stale data being referenced by the first group of read operations. Thus, in the second (deferred update) phase, the old data state is only removed following a “grace period” that is long enough to ensure that the first group of read operations will no longer maintain references to the pre-update data.
FIGS. 1A-1D illustrate the use of read-copy update to modify a data element B in a group of data elements A, B and C. The data elements A, B, and C are arranged in a singly-linked list that is traversed in acyclic fashion, with each element containing a pointer to a next element in the list (or a NULL pointer for the last element) in addition to storing some item of data. A global pointer (not shown) is assumed to point to data element A, the first member of the list. Persons skilled in the art will appreciate that the data elements A, B and C can be implemented using any of a variety of conventional programming constructs, including but not limited to, data structures defined by C-language “struct” variables.
It is assumed that the data element list of FIGS. 1A-1D is traversed (without locking) by multiple concurrent readers and occasionally updated by updaters that delete, insert or modify data elements in the list. In FIG. 1A, the data element B is being referenced by a reader r1, as shown by the vertical arrow below the data element. In FIG. 1B, an updater u1 wishes to update the linked list by modifying data element B. Instead of simply updating this data element without regard to the fact that r1 is referencing it (which might crash r1), u1 preserves B while generating an updated version thereof (shown in FIG. 1C as data element B′) and inserting it into the linked list. This may be done by u1 acquiring an appropriate lock, allocating new memory for B′, copying the contents of B to B′, modifying B′ as needed, updating the pointer from A to B so that it points to B′, and releasing the lock. As an alternative to locking, other techniques such as non-blocking synchronization (NBS) or a designated update thread could be used to serialize data updates. Data element B is partially maintained in the linked list by preserving its pointer to element C. All subsequent (post update) readers that traverse the linked list, such as the reader r2, will see the effect of the update operation by encountering B′. On the other hand, the old reader r1 will be unaffected because the original version of B and its pointer to C are retained. Although r1 will now be reading stale data, there are many cases where this can be tolerated, such as when data elements track the state of components external to the computer system (e.g., network connectivity) and must tolerate old data because of communication delays.
At some subsequent time following the update, r1 will have continued its traversal of the linked list and moved its reference off of B. In addition, there will be a time at which no other reader process is entitled to access B. It is at this point, representing expiration of the grace period referred to above, that u1 can free B, as shown in FIG. 1D.
FIGS. 2A-2C illustrate the use of read-copy update to delete a data element B in a singly-linked list of data elements A, B and C. As shown in FIG. 2A, a reader r1 is assumed be currently referencing B and an updater u1 wishes to delete B. As shown in FIG. 2B, the updater u1 updates the pointer from A to B so that A now points to C. The pointer from B to C is retained. In this way, r1 is not disturbed but a subsequent reader r2 sees the effect of the deletion. As shown in FIG. 2C, r1 will subsequently move its reference off of B, allowing B to be freed following expiration of the grace period.
In the context of the read-copy update mechanism, a grace period represents the point at which all running processes (or threads within a process) having access to a data element guarded by read-copy update have passed through a “quiescent state” in which they can no longer maintain references to the data element, assert locks thereon, or make any assumptions about data element state. By convention, for operating system kernel code paths, a context (process) switch, an idle loop, and user mode execution all represent quiescent states for any given CPU running non-preemptible code (as can other operations that will not be listed here). In some read-copy update implementations adapted for preemptible readers, all read operations that are outside of an RCU read-side critical section are quiescent states.
In FIG. 3, four processes 0, 1, 2, and 3 running on four separate CPUs are shown to pass periodically through quiescent states (represented by the double vertical bars). The grace period (shown by the dotted vertical lines) encompasses the time frame in which all four processes have passed through one quiescent state. If the four processes 0, 1, 2, and 3 were reader processes traversing the linked lists of FIGS. 1A-1D or FIGS. 2A-2C, none of these processes having reference to the old data element B prior to the grace period could maintain a reference thereto following the grace period. All post grace period searches conducted by these processes would bypass B by following the links inserted by the updater.
Grace periods may asynchronous or synchronous. According to the asynchronous technique, an updater performs the first phase update operation, specifies the second phase update operation as a callback, then resumes other processing with the knowledge that the callback will eventually be processed at the end of a grace period. The call_rcu( ) primitive represents one interface that RCU provides for use with asynchronous grace period detection. According to the synchronous technique, an updater performs the first phase update operation, blocks (waits) until a grace period has completed, and then implements the second phase update operation, such as by removing stale data. A primitive known as “synchronize_rcu( )” represents one interface that RCU provides for use with synchronous grace period detection. A related primitive known as “synchronize_rcu_expedited( )” speeds up grace period detection by approximately an order of magnitude, but at the expense of significant CPU overhead.
One variant of read-copy update known as sleepable read-copy update (SRCU) allows RCU readers to momentarily sleep while executing within their RCU read-side critical sections. This is accomplished by refusing to provide asynchronous grace-period interfaces, relying instead on synchronous grace period detection via calls to a pair of primitives known as “synchronize_srcu( )” and “synchronize_srcu_expedited( )” These primitives are the SRCU counterparts to the synchronize_rcu( ) and synchronize_rcu_expedited( ) primitives mentioned above. Unfortunately, these primitives (and especially the latter) can result in significant OS jitter due to the need to coordinate synchronous grace detection with the operations of readers as they enter and leave their SRCU read-side critical sections.
OS jitter represents undesirable variation in the latency of operating system kernel operations that should otherwise be determinative and predictable, particularly where real-time response is desired. A solution that improves SRCU's grace period detection operations would therefore be advantageous. What is needed in particular is an SRCU grace period detection solution that reduces OS jitter without significantly increasing read-side overhead and without significantly limiting scalability.